Explanation:
Perhaps the original
spiral
nebula, M51 is a large galaxy, over
60,000 light-years across, with a readily
apparent spiral structure.
Also cataloged as NGC 5194, M51 is a part of a
well-known interacting galaxy pair, its spiral arms
and dust lanes clearly sweeping in front of companion galaxy
NGC 5195 (top).
This
dramatically processed color composite
combines M51 image data from the
Calar
Alto Observatory's 1.2 meter telescope.
The data include long exposures through a narrow hydrogen alpha
filter that trace emission from atomic hydrogen.
Reddish hydrogen
emission regions, called
HII regions,
are the regions
of intense star formation seen to lie mainly along M51's
bright spiral arms.
Intriguingly, this composite also shows
red hydrogen emission structures
in the faint features
extending even beyond NGC 5195, toward the top of the frame.